The present invention relates to semiconductor structures. It finds particular application in semiconductor laser structures and more particularly to methods of making semiconductor lasers utilizing distributed feedback. It will be appreciated, however, that the invention is also amenable to other like applications.
A semiconductor laser is ordinarily made of Group III-V semiconductor materials. One particularly useful form of such a laser utilizes distributed feedback (xe2x80x9cDFBxe2x80x9d). In other words, optical feedback is generated along the entire cavity of the laser. For example, such feedback is supplied by means of a DFB diffraction grating whose stripes (xe2x80x9cteethxe2x80x9d) run perpendicular to the length (longitudinal direction) of the laser cavity. Such lasers, however, tend to suffer from spatial holeburning (spatial variation in optical gain saturation along the longitudinal direction) and from adiabatic chirping. More specifically, they suffer from relatively low gain near the highly reflecting mirror of the laser owing to spatial variation in gain saturation, and from non-symmetrical spectral intensity distribution around the spectral maximum. In turn, such holeburning and chirping cause, among other things, an undesired lack of single mode operation as well as an undesired lack of linearity of laser response to applied signals.
A DFB laser has certain advantages over a Fabry-Perot (xe2x80x9cFPxe2x80x9d) cavity edge-emitting laser. First, the emission wavelength of the DFB laser is selected by the period of the grating near the active region. Second, optically smooth vertical facets are not necessary in the DFB laser structure. Therefore, the end facets of DFB lasers made from some Group III-V materials, e.g. alloys of AlGaN, may be easier to fabricate than FP lasers made from the same materials. FIG. 1 illustrates a GaN laser diode 10 grown on c-face sapphire 12. In general, an FP laser diode 10 includes an n-layer 14 and a p-layer 16. A vertical facet mirror 18 is formed by etching the n-layer 14 and the p-layer to a depth of about two (2) microns. Output beams 22a, 22b are emitted from the mirror 18. A p-contact 24 and an n-contact 26 are electrically connected to the p-layer 16 and the n-layer 14 (through an n-contact layer 28), respectively. Because of the limited etch depth, the output beam 22b is partially refracted into the substrate 12 as a beam 22c and partially reflected as a beam 22d. 
Recently, there has been much technical effort focused on InGaN based short wavelength semiconductor lasers. Violet, blue, and green InGaN/AlGaN lasers are expected to be especially useful in applications including printing, displaying, and optically storing data. Although long lifetime violet and blue InGaN laser diodes grown on sapphire substrates (using lateral overgrowth techniques on SiO2 masks) have been realized, major issues regarding defect-free metal organic chemical vapor deposition (xe2x80x9cMOCVDxe2x80x9d) growth of InGaN still exist. Also, the formation of laser mirrors is not nearly as easy and straight-forward as in common red and infra-red (xe2x80x9cIRxe2x80x9d) semiconductor laser materials (e.g., GaAs), in which mirrors are easily formed by cleaving along crystal planes. Lasers grown on a grooved substrate, which suggest the use of DFB rather than Fabry-Perot like cavities, are known. Such grooved substrates result in much easier fabrication of high-quality laser cavities having nearly 100% mirror reflectance.
Index coupled DFB lasers have been fabricated for GaN lasers. However, the threshold current densities required for index coupled GaN lasers are relatively high. Until now, gain coupled DFB GaN lasers have not been realized.
The present invention provides a new and improved apparatus and method, which overcomes the above-referenced problems and others.
A distributed feedback structure includes a substrate material. An active layer has an alloy including at least one of aluminum, gallium, indium, and nitrogen. A first cladding layer, having an alloy including at least one of the aluminum, the gallium, the indium, and the nitrogen, is on a first side of the active layer. A second cladding, having an alloy including at least one of the aluminum, the gallium, the indium, and the nitrogen, is on a second side of the active layer. Periodic variations in at least one of the first and second claddings provide a distributed optical feedback.
In accordance with one aspect of the invention, the active layer includes an active region quantum sized in one dimension.
In accordance with another aspect of the invention, the periodic variations are created by changes in a thickness in at least one of the first and second claddings.
In accordance with another aspect of the invention, the periodic variations are created by a periodic dielectric grating structure one of below and above the active region.
In accordance with a more limited aspect of the invention, a buffer material is deposited between the substrate material and one of the first and second claddings. A contact material is deposited on the other of the first and second claddings. A first contact is deposited on the contact material. A second contact is deposited on the buffer material. The first contact electrically communicates with the second contact.
In accordance with another aspect of the invention, a tunnel barrier layer is between the active layer and the first cladding.
In accordance with a more limited aspect of the invention, a high-aluminum content, n-type AlGaN:Si layer is deposited on one side of the periodic variations.
In accordance with a more limited aspect of the invention, the first cladding extends into the periodic variations and contacts the second cladding.
In accordance with a more limited aspect of the invention, a barrier layer is between the second cladding and the periodic variations.
In accordance with a more limited aspect of the invention, the periodic variations extend into the second cladding.
One advantage of the present invention is that it combines the advantages of quantum wires, DFB, and stress relief using a single structure. Another advantage of the present invention is that the crystal growth eliminates some problems that are currently major processing issues.
Another advantage of the present invention is that it reduces crack propagation through the active layer, thereby enabling more efficient electron-hole recombination.
Another advantage of the present invention is that it produces GaN lasers that work at relatively short wavelengths (e.g., less than 430 nm) without being cooled to freeze-out leakage.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.